1. Field of the Invention
The present invention relates to an injection apparatus for use in molding of resin materials, metallic materials, etc., which has a plasticating screw in a heating cylinder, and dispenses with a valve.
2. Detailed Description of the Prior Art
Most of in-line screw type injection apparatuses for use in molding of ordinary resin are provided with a ring valve for preventing a back flow of the measured resin material, at a front end of a screw of the injection apparatus.
After plastication (melting and kneading) by means of the screw, the resin measured and stored in a front portion of a heating cylinder is compressed due to injection advance of the screw, to thereby generate resin pressure. Then, the ring valve is moved backward due to the resin pressure, whereby the front end of the screw is closed. Therefore, the operation of the ring valve is rendered unstable, and the quantity of leakage of the resin is unstable at every injection. Further, a measured quantity of the resin can have an error at every measurement, which adversely affects the accuracy of molded items.
Further, in the conventional injection apparatus, the plasticated resin passes through grooves formed in the screw front end within the ring valve to be stored (measured) in the front portion of the heating cylinder. Therefore, heating from the heating cylinder can be interrupted by the ring valve. Still further, after passing through the ring valve, the resin flows along the front end surface of the screw to a front central portion of the heating cylinder, where external heating is most difficult to reach. This flow of the resin coupled with poor thermal conductivity of the resin per se can pose a problem of temperature unevenness in the measured resin.
To cope with the above-mentioned problems, a number of injection apparatuses which dispense with the ring valve have been developed. However, none of them can overcome both the problems of the degraded measuring accuracy and poor temperature control.
It is therefor an object of the present invention to provide an injection apparatus which dispenses with a valve, and is capable of eliminating uneven measurement of a plasticated molding material, such as a resin material and a metallic material, and temperature unevenness in the material, to thereby always carry out injection molding in a stable manner, only by changing the structures of front ends of a conventional heating cylinder and a conventional screw, respectively.
To attain the aforesaid object, the present invention provides an injection apparatus comprising a heating cylinder having a front end and being provided with a band heater surrounding the same, a front end member arranged at the front end of the heating cylinder, and having a nozzle, a plasticating screw arranged in the heating cylinder in a freely rotating and reciprocating manner, and an injecting plunger connected to the screw in one body and having a conical front end, wherein the front end member has a measuring chamber formed therein at an internal central location thereof, the measuring chamber having a predetermined length, an internal diameter of which is reduced by 8 to 15% with respect to an internal diameter of the heating cylinder, the plunger having a diameter almost equal to the internal diameter of the measuring chamber, thereby being inserted into the measuring chamber in a freely reciprocating manner, a difference between the internal diameter of the heating cylinder and the diameter of the plunger generating a gap functioning as a plastication-accelerating block, on an external peripheral surface of the plunger, the plastication-accelerating block having a distance longer than an injection stroke.
In the injection apparatus as described above, the plunger has a shaft front portion, and the conical front end and the shaft front portion of the plunger have a plurality of flow channels formed therein.
According to the injection apparatus having the construction as described above, the plastication-accelerating block is defined by the gap which is formed on the external peripheral surface of the plunger and between the measuring chamber in the front end member and the plunger. After plastication of the molding material, by virtue of the plastication-accelerating block, the molding material is heated by the heating cylinder over the distance longer than the injection stroke. As a result, temperature unevenness in the material which has been a problem in the conventional injection apparatus can be eliminated.
Further, the plasticated material stored in the measuring chamber in the front end member is primarily compressed by the plunger to measure a predetermined quantity thereof. As a result, uneven measurement due to a back flow of the material can be dispensed with, and the thus measured plasticated material always has a constant density, which leads to further improved injection preciseness. Especially when molded items containing only several cubic centimeters of the injecting material (such as super-precision molded electronic parts) are produced, or when a metallic material which has its viscosity extremely decreased when melted is molded, the injection apparatus of the present invention can advantageously carry out injection under stable measurement of the material.